Natural Rubber is well known to be a tough and nervy polymer. Mastication is a must for natural rubber prior to its compounding. Quite often a peptizer is required to be added to bring down its viscosity. During mixing of the rubber with large amounts of fillers, a plasticizer is essential to ease processing and facilitate dispersion of fillers in rubber. Usually it is know to add plasticizer during compounding either on the mixing mill or in the internal mixer i.e., Banbury, which is cumbersome. The plasticizer is basically hydrocarbon based. Because of the depletion of petroleum based natural sources day-by-day, there is a constant search for alternative resources of material. Renewable resources, which are agricultural and forest based, are supposed to play a significant role as such alternative sources of energy for industrial application in future.
The multi functional additive role of cardanol produced by double vacuum distillation of cashew nut shell liquid (CNSL) a by product from the Cashew Cultivation, which is basically a mix of alkenyl phenols is well known. In particular such Cardanol is known to have the following multi-functional additive characteristics when added to rubber:—    a) The Cardanol act as good plasticiser by way of increasing plasticity and reducing the viscosity of the rubber compounds and aid in processing of the rubber in mixing mills, internal mixers, during calendaring, extrusion and injection molding. This enhances the output rate. Also the torque required is less for the plasticized rubber than that for unplasticised one. Thus power consumption in processing is also reduced.    b) The additive is also known to act as a cure promoter and thus reduces the cure time. This improvement is very significant from the production point of view as it leads to enhanced industrial throughput thus saving labour, power and hence money.    c) The additive enhances mechanical strength properties such as tensile, tear, flex and abrasion resistance. It results in rubber products, which can withstand higher loads and also successfully resist tear during use.    d) The additive is further known to act as good antioxidant and is supposed to assist in air-ageing resistance. Anti-oxidants are a must for the rubber compounds especially for the diene rubbers. Cardanol being phenolic in nature is expected to have certain anti-oxidant properties and are comparable with amine type anti-oxidants.    e) The additive is also known to aid in better filler dispersion in rubber matrix and enhance rubber-filler interaction and consequentially the mechanical strength properties are improved.    f) Moreover, the phosphorylated derivatives of Cardanol(PCP) act as mild fire retardant.
While the above reveal the multi functional additive advantages of CNSL/Cardanol and its phosphorylated derivatives in rubber compounding, there has been some inherent problems in compatibility of Cardanol or its derivatives with respect to rubber. Importantly, it is found that when Cardanol or its derivatives such as PCP are added directly onto the Rubber, on the mixing mill, rubber gets crumbled down and falls down on to the tray. It is thus difficult to handle such material. Moreover, the mixing get delayed, imperfect and there is loss of expensive material. Added to the above, due to such problems of compatibility of such Cardanol in rubber the processing of rubber involve more than usual processing time in the mill which again is complex, time consuming and obviously also cost extensive. Apart from the above complexities involved in the processing of rubber involving the use of Cardanol the incompatibility of the additive when directly added to the rubber base is further shown by the fact that even after physical blending the additive leaches out during storage and use. The aforesaid, therefore, demonstrate the complexities involved in proper and effective use of the multi functional additive role of Cardanol and its phosphorylated derivatives in rubber compounding. Accordingly, therefore, in spite of the presently known multi functional additive characteristics of Cardanol and its phosphorylated derivatives it has not been possible to make effective and proper use of such additive characteristics of Cardanol and its phosphorylated derivatives either in relation to processing and/or obtaining rubber of desired improved characteristics.
The closest related prior art may be given as below:
In the U.S. Pat. No. 1,819,416), M. T. Harvey et al. have claimed the novel use of Cashew Nut Shell Liquid as a physically added additive that improves the moisture resistance of the rubber. Hence the use of such rubber composition in electrical insulation. The present invention deals with chemical grafting of cardanol/its derivative onto the rubber main chain which makes it an integral part of the rubber there by imparting inherent Multi Functional Additive characteristics to the rubber.
The phosphorylated derivatives of cardanol known as PCP (Anorin-38) is found to behave as a multifunctional additives (when physically added to rubber during compounding) by A. R. R Menon, C. K. S. Pillai and G. B. Nando in 1992 (Ref: 1-11)    1) Rheology of phosphorylated cashew nut shell liquid prepolymer modified natural rubber. Menon, A. R. R.; Pillai, C. K. S.; Bhattacharya, A. K.; Nando, G. B.; Gupta, B. R., Kautschuk Gummi Kunststoffe (2000), 53(1-2), 35-41.    2) Cure characteristics and physicomechanical properties of natural rubber modified with phosphorylated cashew nut shell liquid prepolymer—a comparison with aromatic oil. Menon, A. R. R.; Pillai, C. K. S.; Nando, G. B. Journal of Applied Polymer Science (1999), 73(5), 813-818.    3) Modification of natural rubber with phosphatic plasticizers: a comparison of phosphorylated cashew nut shell liquid prepolymer with 2-ethylhexyl diphenyl phosphate. Menon, A. R. R.; Pillai, C. K. S.; Nando, G. B. European Polymer Journal (1998), 34(7), 923-929.    4) Vulcanization of natural rubber modified with cashew nut shell liquid and its phosphorylated derivative—a comparative study. Menon, A. R. R.; Pillai, C. K. S.; Nando, G. B. Polymer (1998), 39(17), 4033-4036.    5) Physicomechanical properties of filled natural rubber vulcanizates modified with phosphorylated cashew nut shell liquid. Menon, A. R. R.; Pillai, C. K. S.; Nando, G. B. Journal of Applied Polymer Science (1998), 68(8), 1303-1311.    6) Self-adhesion of natural rubber modified with phosphorylated cashew nut shell liquid. Menon, A. R. R.; Pillai, C. K. S.; Nando G. B. Journal of Adhesion Science and Technology (1995), 9(4), 443-51.    7) Rheological studies on blends of natural rubber and phosphorylated cashew nut shell liquid. Menon, A. R. R.; Pillai, C. K. S.; Bhattacharya, A. K.; Nando, G. B. Polym. Sect., Editor(s): Bhardwaj, I. S. Polym. Sci. (1994), 2, 657-61.    8) Effect of phosphorylated cashew nutshell liquid on the physicomechanical properties of natural rubber vulcanizates. Menon, A. R. R.; Pillai, C. K. S.; Nando, G. B. Kautschuk Gummi Kunststoffe (1992), 45(9), 708-11.    9) Phosphorylated cashew nut shell liquid prepolymer—a novel multifunctional additive for rubber compounding. Menon, A. R. R.; Pillai, C. K. S.; Nando, G. B. Metals, Materials and Processes (2001), 13(2-4), 179-190.    10) Chemical crosslink density and network structure of natural rubber vulcanizates modified with phosphorylated cardanol prepolymer. Menon, A. R. R.; Pillai, C. K. S.; Nando, G. B. Journal of Applied Polymer Science (1994), 51(13), 2157-64.    11) Thermal degradation characteristics of natural rubber vulcanizates modified with phosphorylated cashew nut shell liquid. Menon, A. R. R.; Pillai, C. K. S.; Nando, G. B. Polymer Degradation and Stability (1996), 52(3), 265-271.    12) U.S. Pat. No. 5,916,850; dated 29 Jun. 1999.    13) US patent No: 2002/0128159 A1; dated 12 Sep. 2002.    14) Charmondusit, Kitikorn; Kiatkamjornwong, Suda; Prasassarakich, Pattarapan. Journal of Scientific Research of Chulalongkorn University (1998), 23(2), 167-181.    15) Thai, Hoang; Park, Jong-Gu.; Tap Chi Hoa Hoc (2000), 38(2), 81-87, 96.    16) John, George; Pillai, C. K. S.; Polymer Bulletin (Berlin, Germany) (1989), 22(1), 89-94.
Multifunctional activity of phosphoridithionic acid derivatives of saturated and unsaturated CNSL has been patented by Krishna Kumar Swamy and Deepak Kumar Tuli et al in 2002 and 1999 respectively (Ref: 12-13). These patents are completely different from what is being claimed in the present patent application.
The knowledge of multifunctional additive of cardanol and its derivatives is new and was revealed by Menon and Nando who proved by research in the last few years that it can perform various functions in rubbers as an additive (Ref: 1-11).
The idea of grafting is novel to this work and was implemented by the inventors. The multifunctional additive term for cardanol and its derivatives was assigned by Prof. G. B. Nando and his coworkers. This idea is novel and akin to Nando and his group. Also the voluminous work on cardanol and its derivatives as monomers, additives and raw materials were carried out by earlier workers from only a chemist's view point. The idea of grafting cardanol and its derivative, thereby generating new grades of rubbers has been carried out from a technologist's view point.
Since cardanol itself doesn't form easily a homopolymer but an oligomer of 4 units, the idea of graft copolymer has been ruled out. But, the potential benefits of grafting single molecules and oligomers of cardanol onto the rubber backbone has been realized by the inventors in the year 1998 and thus this novel idea has been implemented in the Rubber Technology Centre laboratory after grant of this project by from CSIR in the year 2001.
The novelty of the present invention lies in tapping the potential benefits of chemically anchoring cardanol and its derivative to the rubber backbone. We would like to emphasize that no prior art is closer to this idea. The prior art related to cardanol is only about adding physically Cardanol as an additive in rubber compounding or making use of Cardanol in manufacturing cardanol-formaldehyde resins which are used as friction materials. Coming to the prior art related to grafting onto rubber, the literature is replete with examples of preparation of graft copolymers with Methyl methacrylate, styrene and other vinyl monomers onto the rubber. The purpose of making graft copolymer varies from rubber to rubber and monomer to monomer. Till date, no literature is available regarding grafting of cardanol onto rubber.
U.S. Pat. No. 4,526,579 deals with the process for grafting a hydrophilic polymer of a carboxylic acid or a salt thereof onto the surface of a natural rubber article by contacting said article in reaction in inert solvent with a vinyl carboxylic acid anhydride and initiator, and subsequent washing and hydrolysis of anhydride groups to carboxylate groups and an article when prepared by the invention process.
U.S. Pat. No. 4,230,833 deals with single stage process comprising continuously introducing a solution of synthetic rubber in a monomer system comprising methyl methacrylate into a single reactor in the presence of a free radial initiator, continuously stirring said solution to create a turbulent flow, controlling the temperature at about 161 to 195° C., controlling the average residence time to below 90 minutes and continuously removing the resultant graft polymer having a dispersed rubber phase and a continuous hard phase from said reactor.
U.S. Pat. No. 3,954,911 deals with preparation of novel high impact plastics by the inclusion of 2 to 30 weight percent of chlorobutyl rubber in polymers of styrene and styrene-acrylonitrile and the grafting of said polymers onto said rubber. The graft copolymers are obtained by polymerizing the monomers of styrene or styrene-acrylonitrile in the presence of the chlorobutyl rubber, which is preferably precrosslinked.
U.S. Pat. No. 3,944,630 deals with preparation of an impact resistant plastic graft copolymer composition by agglomerating 100 parts by weight in terms of the solids content of a synthetic rubber latex (A) containing small-sized rubber particles by adding 0.1-5 parts by weight, in terms of the solids content of a carboxylic acid containing copolymer latex (B) having a pH of at least 4, said copolymer latex being prepared by polymerizing a mixture of monomers comprising 5-20% by weight of at least one monomer selected from the group consisting of acrylic acid, methacrylic acid, itaconic acid and crotonic acid and 95-80% by weight of alkyl acrylate having 1 to 12 carbon atoms in the alkyl group, in the presence of at least one anionic emulsifier, and adjusting the pH of the mixed latexes (A) and (B) to not less than 6, stabilizing the agglomerated latex with at least one nonionic emulsifier, and grafting 93-30 parts by weight of a monomer or mixture of monomers, which is capable of producing a glassy thermoplastic polymer having a glass transition temperature of not less than 50° C. in the presence of or onto 7-70 parts by weight in terms of solids content of the agglomerated and stabilized synthetic rubber latex.
A French patent No: 1438700 deals with the method of graft copolymerizing olefins onto natural or synthetic rubbers to give polymers with increased transparency, impact resistance, and rigidity and the factors controlling the improvements in transparency and impact resistance are the size of the rubber particles and the presence of a coagulant.
U.S. Pat. No. 5,232,748 deals with the method of grafting monomers or prepolymers onto nonmetallic substrates, e.g., cellophane, comprises activating the substrate by contacting it with a solution of AgNO3 and precipitating. Ag2O or colloidal Ag by an alkali hydroxide, whereby particles of Ag or Ag2O are uniformly distributed in situ throughout the surface of the substrate, contacting the treated substrate with a polymerizable compound containing monomers to effect graft polymerization, and curing the polymer by microwave or laser energy. A cellophane sheet was preactivated by dipping in aqueous AgNO3 and then in aqueous KOH solutions, was then treated with an aqueous solution containing hydroxyethyl methacrylate and acrylonitrile, air-dried, and cured in microwave to give a grafted film showing wt. gain of 20.58%, compared with 2.63% for a similarly grafted film without preactivation.
Thiraphattaraphun et al. (Ref: 14) have reported the graft copolymerization of Methyl Methacrylate and styrene onto natural rubber in an emulsion process at various concentrations of monomers, emulsifier, initiator, and reaction temp. The grafting efficiency, graft ratio, and monomer conversion were reported. Thai et al. (Ref: 15) have reported the graft polymerization of styrene onto ethylene-propylene-diene monomer (EPDM) rubber containing 5-methylene-2-norbornene as the termonomer. The suitable conditions for graft polymerization were found to be a reaction time of 40 h; a reaction temperature of 80° C.; an initiator concentration of 3 wt. %; and toluene as a solvent. Pillai et al (Ref: 16) have grafted Cardanol onto cellulose (filter paper) using BF3.Et2O as carbocationic initiator. The graft yield of 15-25% was reported within 2 h without significant gelation of the monomer. Extensive water repellency was shown by Whatman No. 1 filter paper after the grafting reaction.